In reply to Rod Price.
Hi Rod,
I didn’t want to openly speculate in the article, but yes, the correlation between the boundaries of the blocks and the areas which lost material is interesting. During reentry, the greatest thermal and mechanical stresses should be found at the boundaries between two different components or types of material. I looked through archival photos of the Apollo Command Modules while doing research for this article. The spallation on the CM heat shields was much more limited than on Orion, but it was present. It was restricted to the edges of the heat shield and the areas surrounding the bolts which connected the CM to the Service Module (SM), which supports your hypothesis.
Avcoat was used for the Apollo heat shield. However, the Apollo heat shield was a monolithic (single-piece) component with a honeycomb-like structure. Each cell in the honeycomb was filled in by hand, which was a time-consuming process. When NASA tried to scale up this design for the EFT-1 Orion test flight in 2014, they learned that the larger Orion heat shield is prone to cracking as the Avcoat, which is injected into honeycomb cells as a viscous putty, cures. That prompted them to switch to the block design.
When NASA was designing Orion, they considered an alternative to Avcoat called PICA, which was used for the Perseverance, Curiosity, and Crew Dragon heat shields. They rejected PICA because it would have required a block design and because there were no proven gap fillers to seal the voids between the blocks. Presumably, they had to develop a gap filler when they switched to blocks of Avcoat, but designing a segmented heat shield is always difficult and they could have encountered unexpected behavior. To know for certain, we will need to wait until NASA releases the results of the heat shield investigation, which should happen this fall.
Best regards,
Alex
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